Winter road conditions are often hard to estimate and can be challenging to drivers of vehicles. In particular, at temperatures below 4° C., the current road conditions can often be difficult to estimate, because, for example, icing of roads, or formation of black ice in patches.
To prevent formation of ice, or thaw already formed ice, de-icing salt, also referred to as road salt, for example in the form of dry salt, pre-wetted salt or salt solution, i.e. an aqueous solution of salt, also referred to as brine, is often applied to roads.
De-icing salt usually has a high component of sodium chloride. In addition, for example, calcium sulfate, magnesium sulfate or other minerals may be contained within the de-icing salt. Calcium chloride, magnesium chloride and potassium chloride are also used as an alternative to sodium chloride, or in suitable mixtures, for example.
The action of the de-icing salt is based on molar melting point reduction, a chemical-physical effect, by which as the freezing point of a liquid decreases, the more particles are dissolved therein. Accordingly, the freezing point of a solution depends upon the present salt concentration. Because of a thawing effect and prevention of initial formation of ice on roads, requires that a certain minimum salt concentration must be present to prevent the freezing process at a prevailing ambient temperature. Consequently, it can be necessary to change the amount of de-icing salt, and, thereby, the salt concentration as a function of the ambient temperature.
To date it is not possible to obtain adequate information about the current salt-state of a road. Only in the case in which the driver of the vehicle can directly observe the application of the de-icing salt or is in a position to distinguish salt from ice particles can they derive some information about the salt-state of the road.
In particular, regarding future autonomous driving in self-propelled vehicles, it is however desirable to obtain information about the salt-state of a road independently of the drivers of vehicles, which information can then be passed to the self-propelled vehicle in order, for example, to enable the manner of driving to match the road conditions, and, thereby, increase safety for all road users.
Also, regarding the described dependency of the melting point reduction on the salt concentration, it is desirable to obtain information about the current salt-state of a road.